1. Field of the Invention
The present invention relates to a non-linear distortion compensating apparatus suitable for use in compensating a non-linear distortion generated from an amplifier of a transmitting section of a base station or a terminal station of a wireless communication system, for example.
2. Description of the Related Art
Heretofore, there have been proposed and commercially available several systems as a processing for compensating a non-linear distortion of a transmitting section of a radio communication device. In general, there can be enumerated a negative feedback method, a predistorter method and a feedforward method
The negative feedback method is a method in which an output signal from a transmission power amplifier is again fed back and a non-linear distortion is compensated by a negative feedback circuit. To be concrete, there can be enumerated a Cartesian Loop method for analyzing a feedback signal into in-phase and orthogonal components, by way of example. The negative feedback method encounters a problem of a stability of the negative feedback circuit.
The predistorter method is a method in which a predistortion signal component is added to an input signal in order to cancel a distortion generated from the transmission power amplifier. Although the predistorter method is an open loop control and is excellent in stability unlike the negative feedback method, this method encounters a problem in which the predistortion signal component should be adapted depending upon a characteristic parameter of an amplifier.
The feedforward method is a method in which a distortion generated from a transmission power amplifier is detected and this distortion component is amplified and subtracted from an output signal of the transmission power amplifier. Although this method is the open loop control similarly to the predistorter method and is therefore excellent in stability, this method encounters problems of a non-linear property of a power amplifier added in order to compensate a distortion and a power efficiency.
Of the above three non-linear distortion compensating methods, today, the predistorter method receives a remarkable attention from standpoints of stability and power efficiency. According to the previously-proposed predistorter method, a non-linear distortion characteristic is expressed by an amplitude distortion characteristic and a phase distortion characteristic of a transmission signal and these reverse characteristics are stored in a memory means such as a ROM as data to thereby compensate a non-linear distortion.
Specifically, in the conventional predistorter method, a non-linear distortion is analyzed by measuring a non-linear distortion characteristic of a transmission power amplifier and the non-linear distortion is approximated with a high accuracy by approximation polynomial. Then, a highly-accurate approximation expression of a reverse distortion characteristic is generated from this highly-accurate approximation expression, distortion compensating data is generated by the highly-accurate approximation expression of the reverse distortion characteristic and stored in a suitable means such as a ROM, data is read out from a corresponding address of the ROM in response to the input signal level, and the data thus read out is added to the input signal.
FIG. 10 shows an example thereof. An amplitude characteristic G1 of a transmission power amplifier, for example, is assumed to be such one that an inputted electric power and an outputted electric power are linearly changed up to a certain level and are substantially coincident with an ideal amplitude characteristic and that when they exceed a certain level, they become non-linear characteristics. At that time, if the inputted electric power level, for example, is A, then an ideal outputted electric power level presented at that time is Bxe2x80x2. In actual practice, without compensation, the outputted level becomes B due to the non-linear property of the amplitude characteristic G1. The outputted electric power level can be corrected to become the ideal outputted electric power level Bxe2x80x2 without distortion by adding the predistortion signal in such a manner that the input of the amplifier becomes a level Axe2x80x2.
However, according to the above conventional predistorter method, in order to achieve satisfactory distortion compensating effects, highly-accurate distortion compensating data should be stored in a suitable means such as a ROM. Accordingly, there arise problems that a storage capacity of a memory means that should be required to compensate a distortion should be increased and a circuit scale of a distortion compensating circuit should be increased and as a result a cost has come to increase and an electric power necessary for a processing has to increase unavoidably.
It is an object of the present invention to provide a non-linear distortion compensating apparatus of a simple construction in which distortions can be compensated satisfactorily when distortions are compensated by the predistorter method.
According to the present invention, there is executed a limit operation in such a manner that when a generated predistortion signal is held at a value smaller than a first level, this predistortion signal is not added to an input signal and that when a generated predistortion signal is held at a value larger than a second level which is larger than the first level, this predistortion signal is set to a predetermined fixed value and this fixed value is added to an input signal
With the above arrangement, since the predistortion signal may be generated with respect to an input signal which falls within a constant range, and hence data used to generate a predistortion signal within a range in which a limit operation is executed can be removed. Here, a distortion compensating accuracy can be maintained by satisfactorily setting the first level and the second level.